1. Field of the Invention
The present invention concerns a process for acquiring time-resolved magnetic resonance data as well as a magnetic resonance imaging scanner for that purpose. In particular, the process can be used for contrast enhanced magnetic resonance angiography (CE-MRA).
2. Description of the Prior Art
Magnetic resonance imaging is a frequently used imaging process that is used for medical problems in particular. With the conventional magnetic resonance imaging (MRI), a person or an object that is to be examined is placed in an examination region of the magnetic resonance imaging scanner. A static basic magnetic field (B0), which is as homogenous as possible, is generated in the region to be examined, through which the nuclear spin of the object being examined are aligned with the basic magnetic field. Through irradiation of the object being examined with radio frequency (RF) pulses, the nuclear spins are deflected from this alignment. During a relaxation of the nuclear spin, a radio frequency signal is emitted, which is then detected as a magnetic resonance signal. In this manner, by applying a number of magnetic field gradients, a spatial coding can be obtained. From the acquired magnetic resonance data, an image can be reconstructed, which shows internal details of the object being examined.
In order to obtain a sufficient image quality, it is necessary with conventional methods to acquire the magnetic resonance data with the object being examined in a state of rest over the course of a certain period of time. Enlargement of the area being examined, wherein the basic magnetic field exhibits a sufficient homogeneity, is, however, subject to limitations. With more recent magnetic resonance imaging processes, in order to scan a larger area the scanning table on which the object being examined is placed is moved constantly through the examination region. During this so-called “continuous table movement” (CTM), magnetic resonance data are recorded and reconstructed. In this manner, a seamless image of a large region of the object being examined can be generated from a single scan.
One example of this application is the contrast enhanced magnetic resonance angiography (CE-MRA), wherein a contrast agent is introduced into the object being examined, and whereby the contrast agent bolus subsequently circulates through the circulatory system. Through the movement of the table, the scanning of the magnetic resonance data can follow the contrast agent bolus through the body of the object being examined. An examination of this type may use rapid gradient systems, an automatic table movement as well as TIM (total imaging matrix) technology. In this manner, the contrast agent bolus can be followed from the renal arteries to the soles of the feet with high image quality and without imaging of the venous phase. The TIM technique in combination with a continuous table movement (TIM-CT) allows for a seamless vascular imaging of the object being examined.
For images to diagnose certain diseases, such as a peripheral arterial obstructive disease, the contrast agent may be injected, for example, in both feet of a subject at different points in time. In order to reliably separate arteries from veins, particularly with imaging for a serious disease, a time-resolved acquiring of image data is necessary. As an example, image data from early, middle, and late phases of the contrast agent injection may be acquired in order to separate arteries from veins. For this purpose, it is preferable for the magnetic resonance data to be acquired at a frequency which is as high as possible. In order to improve the time-resolution, so-called “view sharing” methods may be used. A view sharing of this type during continuous table movement however, requires corrections for non-linearities in the magnetic field gradients, and thus is only compatible to a degree.
In particular, the TIM-CT process, which uses a reconstruction time-optimized correction for non-linearities in the magnetic field gradients, cannot be combined with view sharing. The use of a generalized solution for the gradient correction in order to avoid this problem is not practical, as this requires a very long period for reconstruction. Furthermore, it is frequently not necessary to perform a temporal resolution of proximal parts of recorded image data sets, as a considerable delay in the influence of the contrast agent can be regularly observed in the distal regions of the person being examined. When view sharing is combined with the magnetic resonance data obtained with a continuous table movement, a correlation between the temporal and spatial resolution occurs, i.e. in order to achieve a higher temporal resolution, a lower spatial resolution must be accepted.
As a result, there is a need for a for a method acquiring magnetic resonance data that provides both a high image quality as well as high temporal resolution, particularly in the distal regions of the object being examined. The process should make it possible to obtain an image with an improved temporal resolution of the distribution of the contrast agent in a magnetic resonance angiography examination. Furthermore, it is desirable to acquire the magnetic resonance data with a continuous table movement, in order that even with very short magnets, image data of an object being examined of any size may be obtained. It is also desirable for the process to be comprised of a limited number of steps and simple scanning procedures in order that an optimal work flow is obtained. The process should also achieve a reduction in time and costs in comparison with conventional methods.